One of the most important goals of a mobile wireless telecommunication network like the Global System for Mobile communications (GSM) is to provide means for speech transmission between its users.
In a normal call configuration, e.g. between two mobile devices, the speech signal is first encoded in the originating mobile device; the encoding results in a compressed speech signal. In GSM a variety of digital speech coding algorithms are used for the encoding of the speech resulting in different bit rates according to the algorithm. The encoded signal is then sent over the air interface. A transcoder converts the voice channel coding between the GSM coding format and the standard Pulse Code Modulation (PCM) format in communication systems, e.g. converting a 16 kbps signal to a 64 kbps signal, for transmission in other parts of communication networks. The speech signal is transcoded again to a bit rate of 16 kbps in another transcoder, if it is sent through the air interface to the destination mobile device where it is finally decoded. This configuration of the two speech transcoders, which are responsible for the coding and the decoding is called “Tandem Operation”. The double transcoding described causes degradation in the speech quality and is therefore problematic.
The speech quality may be improved if the transcoding of the compressed speech signal is avoided. This approach is feasible if the originating and destination devices are adjusted to use the same speech coding algorithms. Tandem Free Operation (TFO) which intends to avoid the transcoding and raise the quality of the speech transmission has been standardized in GSM. TFO is suitable for transmission of wideband speech e.g. with a frequency range of 50-7000 Hz or more as coded according to the Adaptive Multi-Rate (AMR) standards of the International Telecommunication Union or the 3rd Generation Partnership Project (3GPP). Several bit rates are defined as mandatory in the AMR standards. Without TFO it is not possible to transmit wideband speech in PCM based networks.
Tandem Free Operation as defined by the standard 3GPP TS 28.062 version 7.0.0 (2007-06) is intended to avoid the double speech encoding/decoding during a voice call. When the originating and destination devices are using the TFO protocol and the same speech codec, it is possible to transmit the speech frames received from the originating device to the destination device without transcoding the speech signal.
Before transmission of speech under the TFO protocol a synchronization procedure is performed. TFO messages are used in order to achieve the synchronization in a TFO connection and also throughout the TFO as a means for controlling the operation. After synchronization has succeeded, TFO frames are exchanged and TFO is established. The objective of TFO synchronization is to check among others if the transcoder entities participating in the call are capable of TFO and if the codec and the coding standards match e.g. if a device using the GSM coding standard communicates with another device using the GSM coding standard. The TFO frames can contain information on synchronization, codec type, system identification e.g. GSM, absence or presence of embedded TFO messages, and other information.
The TFO protocol as implemented into the transcoder entities is ruled by a state machine which contains, among others, two states, one where the protocol is in regular operation and one where the synchronization of the transcoders participating in a TFO session has been lost. Transition to the state of regular operation occurs when the transcoder entity acting as TFO endpoint starts to receive TFO frames sent to it by its remote TFO partner. Transition to the state where synchronization has been lost occurs when the TFO endpoint stops to receive consecutive TFO frames from the remote side.
In the case of a handover i.e. of a transfer of an ongoing call between different transceivers or transceiver stations connecting a mobile station to the mobile telecommunication network, a new transcoder entity can be introduced into the call. The new transcoder entity which is handling the call after the handover is not synchronized with the distant transcoder entity. As a result of the loss of synchronization the transcoder entity remaining in the call neither receives frames from the transcoder entity handling the TFO before the handover nor from the new one. Loss of synchronization causes a restart of the speech encoding by the transcoder entity acting as a starting point but in this case the encoded speech is generated starting from narrowband speech and not from wideband speech. This results in transmission of lower speech quality, and the quality may be further affected by other resulting effects like a speech path delay jump, speech distortion, or a fallback from wideband speech to narrowband speech for a period of time.